I remember a program called Tierra that emulated a basic computer. On this virtual machine were seeded a number of programs written in "assembly" language (I'm not sure what language the programs were really expressed in, probbably C but I dunno). The programs would all try to copy themselves into as much memory space as they could. Over time certain programs thrived while others disappeared, and you could introduce mutations from time to time. It had a simple but highly entertaining graphical representation where you could watch the programs battle it out in an epic mortal struggle. The site is here: www.his.atr.jp/~ray/tierra/ -- but from the looks of it it's been abandoned.

Are there any similar programs out there that are in active development?